It would really help if you were to make a video of the issue so we can see exactly what you are talking about. If that isn't possible - does the screen not update constantly during the memory checking process?

This is a short video of the screen after we selected Memory test. Please note that the red '+' sign and the cursor below 'A' in Atom are blinking continuously. These are the only screen-updates (movements) in and the display continues to be as shown in the video until a force reboot is carried out.